In the prior art, synchrotron radiation is probably the best known source available for scientific applications, such as spectroscopy, in the X-ray region of the electromagnetic spectrum. The properties of synchrotron radiation which make it so useful are its spectrum, high intensity, collimation, and time structure. Before the advent of synchrotron radiation, the only sources available for spectroscopy were line sources from conventional X-ray tubes which left large gaps in the spectrum. Synchrotron radiation filled these gaps because of its continuous spectrum, which extends from this infrared to hard X-rays.
Unfortunately, synchrotron sources require massive, costly machines. The electron beam energy must be large (E&gt;2 GeV), and special and costly optics must be built to extract the X-rays from the ring.
In the prior art, transition randiation has been considered as an alternate source of soft and hard X-rays by M. A. Piestrup, P. F. Finman, A. N. Chu, T. W. Barbee Jr., R. H. Pantell, R. A. Gearhart, and F. R. Buskirk in "Transition Radiation as an X-ray source," IEEE, Quant. Elect. vol. 19, pp. 1771-1781, December 1983, and by M. A. Piestrup in patent application 893,977, "A new X-ray source using high density foils."
The X-ray radiation from such a source is similar to synchrotron emission in that it produces a continuous spectrum. In some cases transition radiation can produce more photons per electron than synchrotron radiation. However, synchrotron radiators can produce more total photons than transition radiators because the storage rings used for producing synchrotron radiation can have a much higher current than the linear accelerators used for producing transition radiation. Thus, in general, synchrotron emitters have higher intensity than transition radiators.
The radiation from a transition source is emitted in a conical radiation pattern which diverges roughly as the ratio of the electron's rest energy, E.sub.o, to the electron's total energy E: .theta.=E.sub.o /E. This radiation pattern is shown in FIG. 1. For example, in electron beams of 50 MeV, the apex cone angle would be approximately 10 milliradians and the spot size of the radiation would be approximately 2 cm in diameter at a distance of 1 meter from the source. Thus, the radiation intensity would decrease as one gets farther from the foil stack. Synchrotron radiation comes from a curved trajectory, and hence is smeared in one plane.
In the prior art, reflectivities of X-rays from single single surfaces at normal or near normal incidence are very small. High reflectivities can be obtained using grazing angles of incidence. This is because at X-ray wavelengths the refractive index of the reflecting medium is very close to, and slightly less than, that of the surrounding medium (vacuum)--the conditions under which total external reflection can occur.
Grazing incidence optics have been used to make X-ray microscopes, X-ray telescopes and X-ray waveguides. In most of these applications, the reflecting optics consists of cylinders of revolution of varying diameters with straight or elliptical longitudinal surfaces. For example, soft X-ray microscopes have been used to image biological specimens. Early such instruments used cylindrical grazing angle optics. See Alan G. Michette, "Optical Systems for Soft X-Rays," Plenum Press, New York, 1986, Chapters 2 and 3, pp. 37-94.
The use of critical angle reflection has been applied to the design and fabrication of X-ray waveguides. For example, a hollow air-filled glass capillary tube with a 200-.mu.m bore has been used to transmit soft X-rays a distance of 30 cm. See R. H. Pantell and P. S. Chung in IEEE Jounal of Quantum Electronics vol. QE-14, p 694, 1978.